Use of audio signals for monitoring and remediating spinning irregularides in NMR spectrometers

ABSTRACT

The use of audio signals from rotating samples in an NMR probe in a polarizing magnetic field as part of a nuclear magnetic resonance spectrometer for monitoring and remediating sample spinning speeds is presented.

This application is a divisional application of U.S. Ser. No.08/475,112,filed Jun. 7, 1995 pending.

FIELD OF INVENTION

This invention relates generally to nuclear magnetic resonance (NMR)analytic instruments and, in particular, to measuring spinning speeds ofrotating samples in NMR probes within such instruments by detecting andutilizing the audio information radiated by the rotating sample.

BACKGROUND OF THE INVENTION

Since its discovery in 1946, Nuclear Magnetic Resonance (NMR), hasbecome a powerful analytical tool in studies of gaseous, liquid, andsolid materials.

An NMR measurement is made by determining the energy difference betweennuclear spin states. In order to accomplish this, a sample of thematerial in question is placed in a polarizing magnetic field andexcited by applying a second, oscillating magnetic field in a directionperpendicular to the first steady state field. This is accomplished byapplying oscillating energy radio frequency (rf) energy across a coilsurrounding the sample. The second magnetic field is created bymodulating the current in this coil to produce pulses of a defined form.This second field causes transitions between nuclear spin states whoseenergies are determined by the first field. The energy absorbed by thenuclei during such an excitation or emitted thereby after such anexcitation provides information on the differences in energy between thespin states.

After the application of the radio frequency pulse or pulses, the nucleiof interest produce a radio frequency signal known as a free inductiondecay (FID), which is detectable with a receiver system, where the coilsurrounding the sample is the primary detection element. TraditionalFourier Transform analysis of the free induction decay (FID) generates afrequency spectrum, which contains one or several resonance frequenciesor lines. The positions and widths of these resonance frequencies orlines are determined by a number of influences, such dipolarinteractions, chemical shift, scalar coupling, or quadropolarinteractions on the nuclei of interest over and above that of theprimary polarizing magnetic field. In certain cases the positions orresonance shifts can be partially of completely obscured by scalarand/or dipolar interactions. The accuracy of NMR measurementsadditionally depends upon the physical form of the sample being studied.Highly accurate chemical shift determinations and separation of NMRlines are possible for liquid samples due to the random tumbling andrapid reorienation of sample molecules in solution. This rapidreorientation effectively causes the surroundings of the resonatingnuclei to appear isotropic on the time scale of the NMR experiment. Ifpolycrystalline, powdery, glassy solids, or the like, are studied,however, the observable lines are broadened due to differentorientations of particles with respect to the polarizing magnetic field.

Various methods have been employed to reduce the amount of linebroadening observed in an NMR spectrum for solid samples. The linebroadening can be partially overcome by using magic angle spinning(MAS). According to this technique, the sample is rotated rapidly at anangle of 54.7 degrees with respect to the polarizing magnetic field,ie., the magic angle. This spinning removes so-called first order linebroadening caused by such factors as chemical shift anisotropy, seculardipolar interactions, and first order quadrupolar interactions.

For magic angle spinning experiments spinning rate of the sample shouldbe high in proportion with the strength of the inter-nuclear interactionor spread of chemical shift values, and is typically achieved withspinning rates of order of 1000 to 15,000 hertz (cycles per second) or60,000 to 900,000 revolutions per minute or 1 to 15 khz. Finally, thespin rate must be relatively stable over the duration of the experimentin order not to reintroduce line broadening in the resulting spectrum.

Devices for spinning samples at very high spinning speeds were firstdeveloped in the 1920's and 1930's for various applications includingthe demonstration of Beroulli's law for science students. (J. W. Beams,Rev. Sci. Instr., 1, 667 (1930); J. W. Beams, J. Appl. Phys., 8, 795(1937); E. Henriot and E. Huguenard, Compt. Rend. 180, 1389 (1925); W.D. Garman, Rev. Sci. Instrum., 4, 450 (1933)). The use of these devicesin NMR experiments was published first in the late 1950's (E. R. Andrew,A. Bradbury and R. G. Eades, Nature, Lond., 182, 1659 (1958); I. J.Lowe, Phys. Rev. Lett., 2, 285 (1959)).

Subsequentally there were a number of improvements to the basic spinnerassembly to increase both sample spinning stability and/or samplespinning speed published both in the scientific literature (E. R.Andrew, L. F. Farnell, M. Firth, T. D. Gledhill and I. Roberts, J.Mag.Res., 1, 27 (1969); R. G. Pembleton, L. M. Ryan and B. C. Gerstein,Rev. Sci. Instrum., 48, 1286 (1977); S. J. Opella, M. H. Frey and J. A.DiVerdi, J. Mag. Res., 37, 165 (1980); K. W. Zilm, D. W. Alderman, andD. M. Grant, J. Mag. Res., 30, 563, (1978); B. Schneider, D.Doskocilova, J. Babka and Z. Ruzicka, J. Mag. Res., 37, 41(1980); V. J.Bartuska and G. E. Maciel, J. Mag. Res., 42, 312 (1981); F. D. Doty andP. D. Ellis, Rev. Sci. Instrum., 52, 1868(1981); R. Eckman, M. Alla, andA. Pines, J. Mag. Res., 41, 440 (1980), K. W. Zilm, D. W. Alderman, andD. M. Grant, J. Mag. Res., 30, 563 (1978)) and in U.S. Pat. Nos.4,511,841 to Bartuska et al., 4,456,882 to Doty, and 4,739,270 toDaugaard et al.

The accurate measurement of sample spinning speed is important for anumber of reasons. First, unless the sample is being spun extremelyquickly at the magic angle in proportion to the nuclear interactionsthat are being minimized, the NMR frequencies being observed will bemodulated by the sample spinning and will appear as spinning sidebandsin the NMR spectrum. These sidebands must be identfied as such so thatthey are not interpreted as legitimate NMR resonances. Second, whilespinning sidebands can be used in identifying the sample spinning speedafter a spectrum has been accumulated, it is desirable to know what thespinning speed is prior to start of the experiment particularly when thesample is spinning at the high extremes of a probe's spinning speedspecifications.

Non-NMR methods used for measuring for sample spinning speed, which havebeen described in the literature, are optical and audio methods. Aportion of the sample spinning speed is in the range of audiofrequencies detectable by the human ear. Audio detection of samplespinning speed in a high speed spinning device has been reported byBeams (J. W. Beams, Rev. Sci. Instr., 1, 667 (1930)) and later inconjunction of the use of these high speed spinning devices in thecontext of NMR experiments by Andrew et al. (E. R. Andrew, L. F.Farnell, M. Firth, T. D. Gledhill and I. Roberts, J. Mag. Res., 1, 27(1969)). The methods described by Andrew et al. consisting of listeningto the charateristic fundamental audio note emitted by the rotor andbeating it against a note from a loudspeaker fed from an audio signalgenerator. The authors found that at rates of rotation below about 500Hz, a high frequency audio note at N times the fundamental is heardwhere N is the number of flutes on the rotor. At higher rates ofrotation the note passes beyond the audio range of the human ear and thefundamental note equal to the rate of rotation is then heard. The beatnote between this fundamental and the tone from the loudspeaker provideda sensitive indication of the constancy of rotation. Above a spinningspeed of 6000 hertz this method becomes ineffective due to thelimitation of the human ear.

Audio detection and monitoring of sample spinning has been abandoned infavor of detection of sample spinning via optical methods. Methods areknown for detecting reflected light from a sample container to determinethe sample spinning speed. It is additionally known to use opticalsignals from the spinning samples with computer controlled spinningdevices, which automatically regulate the sample spinning speed duringan NMR experiment. (T. D. Maier and T. Huang, J. Mag. Res., 91,165(1991); J. N. Lee, D. W. Alderman, J. Y. Jin, K. W. Zilm, C. L. Mayne,R. J. Pugmire, and D. M. Grant, Rev. Sci. Instrum., 55, 516 (1984); H.J. M. DeGroot, V. Copie, S. O. Smith, P. J. Allen, C. Winkel, J.Lugtenburg, J. Herzfeld and R. G. Griffin, J. Mag. Res., 77, 251(1988)), Varian data sheet: Spinning Speed Control in MAS Experiments.

There are many reasons for the predominence of optical detection ofsample and the abdandonment of audio detection of sample spinning speed.Foremost, the ease of implementation of optical detection techniques hascaused the use of audio signals generated by the spinning sample to beignored. The analysis of the audio frequencies of a spinning rotor havebeen relegated, therefore, to the analysis of the ear of the NMRspectrometer operator. Additionally, the presence of a polarizingmagnetic field in which an NMR probe is subjected to radio frequencyirradiation precludes the use of most conventional audio signaltransducers, because they contain magnetic or electromagneticcomponents. Recent successful uses of piezoelectric devices for themeasurement of vibrational frequencies in the range of those commonlygenerated by spinning samples in solid state NMR applications have beenpublished recently. One application has been the use of piezoelectrictranducer to record the spinning speeds of two rotors in a doublerotating probe (A. Samosen and A. Pines, Rev. Sci. Instrum., 60, 3239(1989). A second application (S. J. Putterman, Scientific American,February, 1995, p.46.) demonstrated the use of a piezoelectrictransducer in monitoring high frequency sounds emitted by collapsing airbubbles in sonoluminescence experiments. Piezoelectric audio transducersby their nature should be operational in a polarizing magnetic field.The piezoeletric transducer derives its action from the relations foundin certain crystals or specially treated ceramic materials between amechanical strain of the piezoelectric material and the potentialdifferences existing on conductor plates sandwiching the material.

Even though most spectroscopy practitioners use optical detectionmethods for determining the spinning speed of samples spinning at themagic angle, the experienced practitioner additionally listens to thequality of sound produced by spinning sample to determine if the sampleis spinning correctly.

The easiest samples to pack and spin in magic angle containers or rotorsare powdered materials. When the technique of magic angle spinning ofsamples was being developed, powdered materials were studied exclusivelybecause these materials were relatively easy materials to pack and spinin a rotor, particularly as the initial designs of the magic anglespinning probe were somewhat difficult to spin even sometimes evenwithout a sample. As the magic angle probe design improved, the ease ofspinning conventional powder samples led to the interest of studyingsamples which were of less conventional such as beads, gels and otherforms. These nonconventional samples require much more care in packingso that they spin properly. A sample, which is not spinning properly canexhibit several behaviours. It can occasionally "touchdown", that istemporarily oscillate in some manner and after a period of time correctitself to a proper spinning state. Further, the sample can "crash", thatis stop spinning completely or spin/gyrate/oscillate in non-recoverablefashion. Additionally, all samples including well behaved powders aresuceptable to sample "touchdowns" or "crashes" if samples are spun atextremely high spinning speeds. The optical signal recorded duringa"touchdown" or "crash" provides less information than the audio signal.Depending on the motion of the rotor during a "touchdown" or "crash", insome cases the numerical readout of the optical signal can be extremelyoscilatory; in other cases the numerical reading of the spinning speedcan appear quite stable although the numerical reading is of no relationto actual sample spinning speed. To the experienced practitioner it isquite clear from the audio signal produced by the spinning sample thethe sample has had a "touchdown" or "crash". The novice user with morelimited experience or a hearing impaired user may not be capable ofdiagnosing with only his/her ears an incorrecly spinning sample.

This invention pertains to use of audio signals in a useful andautomated fashion in: (1) monitoring the spinning frequency of thesample during the NMR experiment, (2) adjusting the bearing air if a"touchdown" or "crash" of the rotor occurs, and (3) a shutdown of thesample spinning and termination of the NMR experiment if the abovedescribed actions are unsuccessful. This invention pertains to the useof audio transducers, which are compatable with their use in polarizingmagnetic fields and to radio frequency irradation for sample in saidmagnetic fields.

This invention is a benefit to both experienced and novice users ofmagic angle spinning NMR probes, because it allows for the use of thespectrometer in an automated fashion.

This invention is of great importance to both the experienced user andthe novice user by preventing damage to the magic angle spinning probewhen the sample is both rotating and precessing due to a rotor "crash"by terminating the gas supply to the spinning sample.

BRIEF DESCRIPTION OF THE INVENTION

The present invention exploits the production of audio frequencies byspinning samples in magnetic fields to provide information about thefrequency of the sample rotation during the NMR experiment, the qualityof the spinning of the rotating sample, and utilizing a feedbackmechanism for the correction for an ill-spinning sample, and finallyinitiating appropriate shut-down procedures of both the NMR experimentand the sample spinning if the sample spinning could not be correctedand matched to predefined spinning criteria.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic of NMR apparatus of the invention.

FIG. 2 shows a rotor controller pneumatics block diagram.

FIG. 3 shows a rotor controller connection diagram.

FIGS. 4a-4c shows a rotor assembly for the present invention

FIGS. 4d-4e shows a stator assembly for the present invention.

FIG. 5 shows a frequency spectrum of the audio signal when the sample isspinning smoothly.

FIG. 6 shows a frequency spectrum of the audio signal when the samplehas "crashed".

FIG. 7 is a schematic of a feedback circuit coupled to the audiodetector measuring rotating sample spinning speeds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Portions of an NMR apparatus for practice of the invention are describedschematically in FIG. 1. An NMR spectrometer comprises a polarizingmagnet 103 providing an essentially homogeneous magnetic field intowhich a probe 101 is positioned. At least one radio frequencytransmitter 107 and at least one receiver 113 and digitizers 111 areconnected to the probe 101 via a transmit/receiver switch/preamplifier105. An acquisition controller 109 is connected to the radio-frequencytransmitter(s) 107 in order to control the sequences of radio frequencypulses produced by the transmitter(s) 107 and applied to the probe 101.A data processor 115 is connected to an output of the aquisitioncontroller 109. The data processor 115 is capable of storing, processingand displaying magnetic resonance signals detected by the receiver 113and digitized by the digitizer 111. It is common practice in highresolution NMR studies to cause the sample to undergo a mechanicalrotation about a selected axis. Thus the probe 101 will include apneumatic apparatus for maintaining a gas bearing and a gas actuatedrotor to which the sample is secured. Additionaly, the speed of samplerotation is typically detected optically and displayed in a fashionuseful to the practitioner.

The amount of gas that is supplied to the probe is controlled as shownin FIG. 2, where a computer controlled electro-pneumatic regulator 201and a control valve 202 are placed in series with the drive pressureregulator 203.

In a commonly used mode of operation for an NMR experiment the userspecifies the desired spinning speed and a control algorithm regulatesthe gas flow through the electro-pneumatic regulator 201 and valve 202as described in FIG. 2 to achieve the required spinning speed. FIG. 3shows a connection diagam for controlling and regulating rotor speedswhere the spinning speed is optically detected. The rotor speed issensed by detecting the rotor pulses generated in the pneumatics andtachometer box 301 and the transmitting of the signal to a computer 302where N rotor pulses are used to gate a counter operating at 1 MHz. Inorder to ensure an accurate measurement, the number of rotor periods Nis optimized automatically in the control software. The desired spinningspeed for the sample is entered into the computer 302.

The audio experiments were carried out on a representative rotor andstator as described below. The spinner unit is shown as disassembledconstituents in FIGS. 4a-e. The rotor assembly comprises the end cap 10and the rotor body 12. The rotor body 12 has a cavity 14 to contain thesample therein and is closed by insertion of the end cap 10. The lengthof the rotor body may include a portion 19 which may be arranged tocooperate in a known manner with a light source and light sensor, tomonitor spinning rate during operation.

The stator assembly comprises three pieces as shown in FIGS. 4c-e. Thetop piece (top) 20 and bottom piece (bottom) 22 each contain twelveradially directed jets 24. These jets each comprise 0.3 mm aperatures.Stator body 26 contains a bore 28 with ports 30 to communicate withannular regions 32 to provide pressurized gas to radial jets 24 forstabilizing the axis of rotor body 12 in operation. When assembled, topand bottom 20 and 22 define bore 34 through which rotor body 12 isinserted for operation. An access port 35 is cut into the stator body 26to permit insertion of the coil or coils employed for exitation anddetection of nuclear magnetic resonance in the sample contained withinthe rotor body 12.

Top piece 20 has an inner conical surface 36 which accepts the outerconical profile 38 of end cap 10. Twelve jets 40 are distributedannularly on the inner conical surface 36 to provide pressurized gas,which acts on flutes 42 of the rotor assembly for levitating androtating the rotor assembly. The pressurized gas for jets 40 isfurnished form bore 44. These jets comprise aperatures of 0.4 mmdiameter.

In typical operation, the gases in the respective bores 28 and 44 aremaintained at different pressures. In a usual arrangement for such dualjet spinner apparatus, the rotation, or drive pressure may be severalbars greater the the radial bearing pressure.

In the case that the sample in the rotor is packed properly the airpressure from both the drive and bearing air jets allows for thelevitation and rotation of the sample. However, it is possible for thesample to be initally incorrectly packed in the rotor or during thecourse of the NMR experiment for the sample to become unbalanced. Theresult of either of these situations is a rotor "crash" or a severeimpact of the rotor assembly against the stator. "Crashes" can causedamage to the stator and can entail the removal, disassembly, andreplacement of portions of the the stator assembly.

For the described results, the audio transducer 60 was placed on thebottom portion of the bottom piece 22 of the stator. Without disassemblyof the stator this was the closest that the audio transducer could beplaced to the bearing jets. The audio transducer can be placed anywhereon the stator, but preferably as close to the bearing jets as possiblein order to (1) reduce the interference of white noise of the gas flowsas they flow into the stator and (2) in order to be able to detecthigher signal-to-noise frequencies other than fundamental harmonicsresulting from a rotor "touchdown" or "crash".

FIG. 5 shows a representative trace of an audio signal which has beenFourier transformed resulting in a trace which plots frequency on theabscissa (x-axis) and frequency intensity on the ordinate (y-axis) ofthe audio signal transmitted from the piezoelectric microphone attachedto a stator as described in FIG. 4, through an audio amplifier circuitwith an output suitable as input into a Hewlett-Packard spectrumanalyzer, where the sample is spinning smoothly. The example shown is ofthe sample spinning smoothly with neither precession or oscillation ofthe sample as judged by (1) simultaneous speed detection via opticalmethods and (2) by an experienced user listening to the tone of spinningsample. The sample speed for this example was 4 kHz. This samplespinning speed is approximately midrange of the specified spinning speedfor the probe in use. In addition to the fundamental frequency of thespinning rotor the second, third, fourth, and fifth harmonics wereobserved. As with most sound sources, the spinning sample does notvibrate in just one mode. Several modes of vibration have been set upsimultaneously, generating a plurality of sound frequencies. The lowestfrequency is the fundamental frequency. The higher frequencies are theharmonics. It is known that in the analysis of musical tones therelative intensities of the harmonics contribute in different ways toquality (harshness, brightness, etc.) of the sound produced. It has beenobserved that different samples spinning at the identical sample speedcan vary in loudness and in quality of sound. A well packed powdersample will generally have the most quiet and pleasing to the human earspinning sound. On the other hand, a sample of beads, for example, anirregularly shaped sample with voids within the sample will have theopposite quality of sound from that of the well packed powder.Potentially a spinning instability may be foreshadowed by observingchanges in the relative intensities or positions of various harmonics.

FIG. 6 shows a representative trace of an audio signal where the sampleis not spinning correctly which has been Fourier transformed resultingin a trace which plots frequency on the abscissa (x-axis) and frequencyintensity on the ordinate (y-axis) of the audio signal transmitted fromthe piezoelectric microphone attached to a stator as described in FIG.4, through an audio amplifier circuit with an output suitable for inputto a Hewlett-Packard spectrum analyzer. For the example shown, thesample spinning instability was induced by lowering the bearing pressureuntil the pressure was insufficient for sample levitation. The rotor isboth spinning and precessing in the stator. The spinning instability wasfurther demonstated by an observed oscillation in the bearing gaspressure as the rotor would touch down onto some of the bearing jets.Additionally and very important is that the optical signal recordedduring a "crash" provides less information than the audio signal.Depending on the motion of the rotor during a "touchdown" or "crash" insome cases the numerical readout of the optical signal can be extremelyoscillatory; in other cases the numerical reading of the opticallyderived spinning speed can appear quite stable although the numericalreading is of no relation to actual sample spinning speed.

The resulting frequency spectrum recorded in FIG. 6 during the "crash"is notable for its poorer signal-to-noise relative to the trace shown inFIG. 5. These data demonstrate the importance of placing audio detectiondevices as close to the bearing jets as possible. The main differencesbetween the trace shown in FIG. 5 and that shown in FIG. 6 is that thepreviously seen fundamental frequencies and higher order harmonics haveall disappeared and a set of lower frequencies have been recorded. Forthis particular situation the sample was restored to its stable spinningmode when the bearing pressure was increased. The frequency spectrumrecorded after the bearing correction was identical to that shown inFIG. 5.

So far only the detection and recording of the audio signal of spinningsamples have been demonstated. As with the optical signals from spinningsamples which has been harnessed to provide both useful and regulatoryinformation, so too can the audio signal be used to provide useful andregulatory information and as outlined in FIG. 7. In the presentinvention audio signals are used to regulate sample spinning via acomputer controlled electro-pneumatic regulator and control valve placedin series with both the drive pressure regulator and the bearingregulator resulting in computer controlled drive gas 708 and computercontrolled bearing gas 709. The independent control of the bearing anddrive pressure allows small corrections of the bearing pressure to occurin the case of the detection of small spinning instabilities by theaudio detector 703. In the case of catastrophic "crashes", commands canbe issued from a data processor 706 to first terminate the NMRexperiment and to subsequentally termintate the sample spinning by firstcutting off the gas supply to the drive gas 708 and then cutting off thegas supply to the bearing gas 709.

However, the previously described set of regulatory actions cannot occurunless additionally, the audio signal, monitored by the audio transducerin the NMR probe and coupled to an appropriate audio amplifier, isdetected and processed via a receiver 704 and digitizer 705. Processingof the audio signal by the data processor 706 to the receiver704/digitizer 705 can then occur and the digitally processed storedsignals can be used for interpretation and display by the data processor706 as shown in FIG. 7. Interpretation of the processed audio signal maybe accomplished in a number of ways, such as comparing peak intensities,or peak searching, or other processes well known to practitioners of theart form. If instabilities in rotor spinning or rotor "crashes" aredetected, corrective action is specified by the data processor 706 tocomputer gas flows 708 and 709 to the spinning rotor. Potentiallydifferent ways to correct different spinning instabilities might beincorporated based on analysis of harmonic distributions, intensitiesand/or other spectral features. Several examples follow. (1) As the NMRexperiment proceeds (which can be anywhere from minutes to hours) thesample spinning speed is monitored. Computer controlled pnuematics canchange the bearing pressure to minimize the non-desired frequencies aspredefined for the particular sample spinning criteria and demonstratedin FIG. 5 and FIG. 6. (2) Changes in the fundamental spinning frequencyare additionally monitored and if there is a drop in spinning speed thedrive gas speed is increased. (3) A detected drastic change in spinningspeed can result in the orderly shutdown of both computer controlledbearing and drive pnuematics and also result in the termination of theNMR experiment. This process would eliminate any danger of probe damageand would also eliminate the further acquisition of large amplitudenoise data when coupled with instructions to the NMR apparatus to ceasedata acquisition.

Many modifications and variations of the embodiments described hereinwill be apparent to those ordinarily skilled in the art. Suchmodifications and variations will still be within the scope of theinvention. Therefore, the invention should not be limited by the scopeof the embodiments described, but only by the claims which follow:

What is claimed is:
 1. In an NMR spectrometer for acquiring magneticresonance information, a method for preventing damage to an NMR probe,said probe incorporating a gas driven, gas bearing supported spinningsample, comprising the steps of,(a) acquiring an acoustic spectrum ofsaid spinning sample, (b) comparing selected features of said acousticspectrum to pre-defined criteria to ascertain differences therebetweenwhereby irregularities of spinning of said spinning sample arerecognized, and (c) adjusting the gas pressure at said gas bearing inaccord with said differences to reduce same.
 2. The method of claim 1wherein said step of adjusting includes allowing said irregularities todiminish within a predetermined time interval.
 3. The method of claim 2wherein said irregularities do not diminish within said predeterminedtime interval further comprising reducing and eliminating pressure atsaid gas drive and reducing and eliminating gas pressure at said gasbearing.
 4. The method of claim 2 wherein said irregularities do notdiminish within said predetermined time interval, further comprisingceasing acquisition of said information.
 5. Apparatus for remediation ofspinning irregularities in an NMR spectrometer for the aquisition ofmagnetic resonance information, comprising(a) an NMR probe comprising anair bearing supported sample, and a valve communicating with saidbearing for regulating the pressure at said bearing, (b) an acoustictransducer disposed proximate said spinning sample for acquiring theacoustic waveform thereof, (c) a spectral analyzer for operation on saidacoustic waveform to obtain at least selected spectral features thereof,(d) a comparator to quantify a difference between at least one saidselected feature and a corresponding predetermined criterion and togenerate a signal responsive to said difference, whereby a spinningirregularity is detected and quantified, (e) valve actuator responsiveto said signal for adjusting one or both of said bearing pressure ordriving pressure to reduce said difference.
 6. The apparatus of claim 5further comprising a supervisory processor to determine whether saidspinning irregularity has been removed within a predetermined timeinterval.
 7. The apparatus of claim 6 wherein said supervisory processorcommunicates with said valve actuator, whereby said spinning isterminable in a predetermined manner.
 8. The apparatus of claim 6wherein said supervisory processor comprises a switch to terminate saidaquisition of information.